I've been carrying Arai helmets since 1998 and yours is a very good question. One that most never ask. Most simply buy the helmet that looks the best without regard to protection. If the helmet does not fit correctly, your protection is quite small.
Being in the fire service, I went to more than one motorcycle accident where the rider was over here and their helmet was over there. Some lost their helmet upon initial impact. That is because 70% of riders are wearing a helmet at least 1 size too large. People do not like a tight fitting helmet. But if it does not fit tightly, there is a very good chance that it won't stay on your head in an accident. Sounds absurd, but is a fact.
The truth is, once you get used to a correctly fitting helmet, you'll never go back. There is also a correct way to don your helmet. Done wrong, it will want to shear your ears off. Done right, it's a breeze.
If you shake your head and your helmet does not stay centered on your face, it's too big. If you can grab your helmet at the base at the back, pull up and forward and it starts to come off your head, it's too big. 70% represents a great many riders.
You can get an idea of your head shape by just looking at it. There is no perfectly round head so a Round Oval helmet will still be somewhat longer front to back than side to side. But as the front to back ratio increases, you come to the Intermediate Oval head shape which is where the lion's share of heads reside. A further increase in the front to back ratio moves you into the Long Oval or Football head shape.
A new helmet should fit tight as the padding will conform to the shape of your head after a few hours of wearing it. If the helmet is a 'Perfect' fit new, it will be too lose when the padding settles in. When I get a new helmet, I sit and watch TV with it on my head. You can imagine the ribbing I get from family. But this allows me to find any 'Hot Spots' that might need adjustment before I get out on the road with no opportunity to make any necessary changes.
Going to a dealership and trying on helmets is not a bad idea. However, be aware that they are there to sell you a helmet. They may not be all that knowledgeable. And it is hard to find a dealership that carries good quality (more expensive) helmets because that's a lot of capital tied up in inventory.
It is also a good idea to get a helmet that has replaceable cheek pads and headliner. Many do these days, but not all. You don't need the most expensive helmet out there to get good protection. But it's not a good idea to cheep out on a helmet. Cheaper helmets will not give you the same protection and tend to break down quickly and need replacement. A good helmet will last a long time.
You can tell a lot about a helmet's quality by the materials used in the shell.
And I always put in a plug for full face helmets. For 1 very good reason. If you're wearing a helmet for protection, you're giving up a lot with a 3/4 (0r less) version.
sharp.dft.gov.uk
